1. Field of the Invention
The present invention relates to a silicon single crystal wafer for a particle monitor, which is used as a semiconductor material, and more specifically to a silicon single crystal wafer for a particle monitor, which has an extremely small surface density of light point defects (hereinafter referred to as “LPDs”) on the wafer surface, even when it is repeatedly used.
2. Description of the Prior Art
As a method for producing a silicon single crystal which is used for a silicon wafer of semiconductor material, the Czochralski method (hereinafter referred to as “the CZ method”), the floating-zone method (the FZ method) or the like is employed. Of these methods, a method for growing a single crystal with the CZ method is most frequently employed.
FIG. 2 is a schematic sectional view of a conventional apparatus for producing a silicon single crystal with the CZ method. A method for growing a single crystal by utilizing the single crystal producing apparatus shown in FIG. 2 will be described as follows:
A polycrystalline silicon material is put in a quartz crucible 3 which is mounted in a graphite crucible 4 surrounded by a thermal insulator 6, and then the silicon material is heated by a heater 5 to form a molten silicon material in the quartz crucible. Thereafter, a seed (seed crystal) 1 is immersed into the molten silicon material, and the seed is pulled up in the state in which the seed and crucible are rotated. By the pulling a silicon single crystal 2 is grown in a state in which it is surrounded by a thermal shield 7. In this case, the crucibles 3 and 4 are rotated either in the same direction as that of the single crystal or in the direction opposite thereto.
In the growth of the single crystal 2 by the CZ method, a seed diameter reduction, i.e., the process of reducing the diameter of the seed crystal, is carried out in order to obtain a dislocation-free single crystal, and subsequently the shoulder part is formed so as to grow the single crystal having a predetermined diameter at the body portion, i.e., the body diameter. After altering the shape of the shoulder, the single crystal having a fixed body diameter is grown. After growing the single crystal having a predetermined length, a tail portion is formed in the single crystal such that the diameter thereof is reduced. Then, the process of growing the single crystal 2 is finished.
Wafers for an integrated circuit device of semiconductor are prepared by slicing the silicon single crystal thus produced. In particular, the design rule of 0.13 um size (the pattern width is 0.13 um) is applied to the current devices, and the quality demands of silicon wafer for producing such a device becomes significantly severe.
In the process of producing devices according to the modern device rule as above, a very severe control of particles on the wafer surface is carried out. In this case, the number of particles is measured by a particle counter, and it is required that the surface density of particles becomes extremely small.
In order to produce a silicon single crystal wafer for a particle monitor including a small amount of the COPs (crystal originated particles) regarded as crystal defects, the speed of growing the single crystal is decreased to confine annular oxidation-induced stacking faults (hereinafter referred to as “annular OSFs”) in the single crystal. Thereafter, the wafer can be prepared from the single crystal by selectively slicing it at areas in which no COPs are generated on the surface.
However, a decrease in the growth rate of a silicon single crystal causes the productivity to be reduced, and thereby the cost of manufacturing a single crystal to be increased. Accordingly, this method cannot be employed in the manufacture of a silicon single crystal for a particle monitor.
Furthermore, a method is disclosed for decreasing the number of particles which may be counted by a particle monitor, together with a decrease in the size of COPs by doping a single crystal with nitrogen during the crystal growth.
For instance, Japanese Patent Application Laid-open No. 2000-53489 discloses a method for producing a silicon single crystal wafer for a particle monitor, wherein the single crystal is grown with the CZ method by doping a single crystal rod with nitrogen at a nitrogen concentration of 1×1010-5×1015 atoms/cm3.
In accordance with an example reported in the above specification, it is shown that, for a 6-inch diameter wafer which is cleaned for 60 min with the Standard Cleaning-1 (hereinafter referred to as “SC-1”), which is made by using alkaline chemical liquid mainly containing NH4OH, H2O2, and H20 the surface density of particles having a diameter of not less than 0.13 um is about 1200 counts/cm2 when nitrogen is not doped, whereas it becomes about 1/20 of the above surface density when nitrogen is doped. In accordance with the description of the example, it is estimated that the surface density of particles having a size of not less than 0.13 um is not more than 60 counts/cm2. In recent years, a wafer having such a greater surface density as in this estimation can hardly be used as a wafer for manufacturing devices.
Since a wafer for a particle monitor is repeatedly used in the current process of manufacturing devices, it is desirable that the wafer still has a small surface density even after repeating the cleaning. However, in the conventionally used wafer for a particle monitor, it is found that, even if the wafer has a small surface density in the initial usage, the surface density increases in the repeated usage.
It is conceivable that this problem results from the following two facts [1] and [2]:
[1] When COP having such relatively small size as undetectable with the particle counter on the wafer surface in the initial stage is repeatedly cleaned, the particle size of COPs increases due to the etching effect in the cleaning to form a pit which can be detected as LPD by the particle counter, thereby enabling the count of particles to be increased.
[2] The wafer is subjected to various heat treatments in the course of repeated usages, so that oxygen precipitates are generated on the wafer surface to eventually provide crystalline defects due to oxygen, i.e., BMDs (bulk micro defects). These defects are detected by the particle counter, thereby causing the surface density of particles to be increased.